Despite all that old talk about Mars and Venus, men and women are much more biologically alike than not. But differences in the way our brains are built shed light on everything from the way we flirt to the way we fight to how we raise our boys, says neuropsychiatrist Dr. Louann Brizendine in her provocative new book, The Male Brain. The author talked to TIME about sex, the daddy brain and why some men may be built to cheat.

You immediately address the stereotype that guys have one-track, sex-crazed minds. Biologically speaking, is it true?
I think that's probably more emblematic of the female experience of the male than what's actually going on in the male brain. Certainly the male brain is seeking and looking for sex. But it is also very much seeking and looking for partnership and for choosing "the one."
(See "Female Sexual Dysfunction: Myth or Malady?")

You say the "area for sexual pursuit" is 2.5 times larger in the male brain than in the female brain. Do you worry that people will read that and decide your book confirms the stereotype?
I think there is a kernel of truth in stereotypes. But [understanding human biology] doesn't give males a pass on being civilized or any parent a pass on having to train their sons.

You write that sex and love are linked. How?
The sexual circuitry releases huge amounts of dopamine. The reward system in the brain basically gets triggered during sex and orgasm and then feeds back on the rest of the brain, making it want to do that again and again  and wanting to seek out the person that you're having that lovely experience with again and again. So at some point, the love circuits and the sex circuits get gradually bound together. The sexual part of that experience gets more and more attached to that [particular] female, and gradually merges with that circuitry and identifies that person as "the one." Not all men get that, as we know, but the majority of men do.
(See the top 10 medical breakthroughs of 2009.)

Let's talk about the ones who don't. You say that one gene in particular  which scientists first started studying in voles  may play a role in infidelity.
It's called the vasopressin receptor gene. The prairie vole, which is monogamous, bonds with one female for life, even if he's presented with other, fertile females. His cousin, the montane vole, is kind of a hit-and-run guy. He doesn't stick around at all. Scientists found that the montane vole had a short version of the vasopressin receptor gene, and the monogamous one had a long version of it. They then took the [long] gene from the monogamous one and injected it into the brains of the promiscuous one  and the promiscuous one became monogamous.

In humans they have identified, so far, about 17 different lengths of [the vasopressin receptor gene]. There are several studies that have shown that those males with the longer version are more likely to be married, and their wives are more likely to say they have a happy, successful marriage and there hasn't been any infidelity. The ones with the shorter ones are more likely to be bachelors.

Doesn't suggesting that a propensity to cheat is hard-wired in some guys give unfaithful husbands the perfect excuse?
I don't think it lets you escape responsibility, but I think it lets one honor that underlying impulse and then realize why it's so important to have all the religious and social principles that we're raised with. No matter what [a boy's] genes are, we need to be laying out good role models for how one behaves in one's life. I feel very strongly: this is not an excuse for men to behave badly. But it is something to help men have a deeper insight into themselves, and women to have a deeper insight into men.
(See five paths to understanding the brain.)

You write that men and women process emotions differently. How?
The mirror-neuron system [MNS] allows us to [see a facial expression] and know what that person is feeling. When we are looking at an infant or another person we care about, women will resonate with that feeling a lot longer than men. This is not to say that men don't do this. They do. They start out very quickly in the MNS and get a quick flash of what's going on. Then they switch into another system called the temporal parietal junction system, which allows them to start Google-searching their entire brain circuit for ways to fix the problem.

This type of interaction goes on lots and lots between the couples that come to my office: she just wants him to talk to her about how she's feeling about something before he launches into giving her the solution. And he feels like, well, what good will it do just to wallow in the feelings? I think one of the things that women don't focus on or appreciate is that our men really want to make us happy. He's the fix-it man. He really does want to be our hero, and that's how he expresses his love.

What happens when a guy is becoming a father?
The hormone testosterone is going down and the hormone prolactin is going up in the male brain, because he is smelling the pheromones of his pregnant wife. Prolactin is the hormone in females that makes breast milk. We don't know what it's doing in males yet. We assume it has something to do with making the daddy-brain circuits. By the time the baby is born, he's able to hear infant cries much better. So something about his auditory-perceptual system has actually changed. His sex drive has gone down along with his testosterone. Therefore his brain is being primed to be a caretaker. If he doesn't get some alone time [after birth] with the baby, however, the daddy brain won't develop fully.

You think both men and women have deep misunderstandings of what drives the opposite sex. What are the biggest?
I think the biggest is that all men want is sex. The equivalent for women is that we are all emotional, and all we want is commitment.